1. Field
The following description relates generally to wireless communication and, more particularly, to frequency selection and transition over white space.
2. Background
White space refers to frequency spectrum that is not used by any licensed technology/primary user in many geographical locations. In the absence of a primary user, this frequency spectrum is available for use by any other user (i.e., secondary user) using any technology. A wide area network (WAN) deployment for secondary usage over white space frequencies (or cognitive frequencies) is fundamentally different from more traditional deployments because of two basic reasons: a) the same frequency (or channel) may not be available at all locations because of the presence of a primary user (such as TV signal or wireless microphone); and b) the same channel is shared by various devices using disparate technologies creating interference pattern that changes spatially and temporally. Even in local area network (LAN) deployments, the devices using disparate technologies can significantly interfere on white space over lower frequencies due to propagation characteristics over such frequencies. As an example, in the traditional WLAN deployment over 2.4 GHz frequencies, the signal attenuates significantly over long distances and through walls thereby allowing different secondary users to coexist in their coverage area. However over the VHF/UHF band, the signal attenuates very little over large distances and through the walls thereby creating significant interference between secondary users.
This compels a WAN/LAN to use frequencies that are locally available and have less interference from other devices. In a wireless WAN/LAN (WWAN/WLAN) environment, each base station (BS) (or Access Point) can be considered as a master device selecting a frequency that is best suited for the mobile stations in its vicinity. Each BS has a downlink (DL) coverage area which is the neighborhood around the BS where a mobile station (MS) when present can decode a minimum rate transmission from the BS and uplink (UL) coverage area where the MS when present can transmit at a minimum rate to the BS. The DL and UL coverage areas are a function of selected frequencies and the interference pattern resulting from unknown devices sharing the same frequencies.
Conceptually, Dynamic Frequency Selection (DFS) is a way for a master device to select an available channel where low interference is observed. However, a practical design of a DFS protocol presents various challenges because of the limited information available to entities involved in the DFS decision process.